A fuel cell system is a power generation system that directly converts chemical energy of a fuel into electricity. A fuel cell system includes a fuel cell stack that generates electricity, a fuel supply unit that supplies the fuel cell stack with fuel (i.e. hydrogen), an air supply unit that supplies the fuel cell stack with air (i.e. oxygen) serving as an oxidant that is needed to cause an electrochemical reaction, and a heat-and-water management unit that discharges heat out of the fuel cell stack and controls the operation temperature of the fuel cell stack. A fuel cell stack produces electricity through an electrochemical reaction between hydrogen (fuel) and oxygen (air) and also generates byproducts (heat and water) that need to be discharged out of the fuel cell stack.
A fuel cell stack that is suitably used for a fuel cell vehicle includes many single cells arranged in a row. Each single cell includes a membrane-electrode assembly (MEA) disposed in the center. An MEA includes an electrolyte membrane that allows protons to pass therethrough. Catalyst layers serving as a cathode and an anode at which hydrogen and oxygen react with each other are provided on respective surfaces of the electrolyte membrane. Gas diffusion layers (GDL) are disposed on the surfaces of the catalyst layers. Separators with respective flow fields (channels) through which fuel and air are supplied to the anode and the cathode are disposed on the surfaces of the GDLs. End plates are disposed at respective ends of a single cell to firmly combine the all elements.
In a fuel cell stack, hydrogen and oxygen are ionized through chemical reactions by catalyst layers. Then, an oxidation reaction occurs to generate protons (hydrogen ions) and electrons at a fuel electrode to which hydrogen is supplied, and a reduction reaction involving hydrogen ions and oxygen ions occurs to produce water at an air electrode to which air is supplied. A typical electrode catalyst that is used for a fuel cell is composed of a catalyst support made from a carbon material and a cocatalyst such as Ru, Co, Cu, or the like. Hydrogen is supplied to an anode (also referred to as “oxidation electrode”) and oxygen (air) is supplied to a cathode (also referred to as “reduction electrode”). Hydrogen supplied to an anode is split into protons H+ and electrons e− by catalysts on electrode layers disposed on respective surfaces of an electrolyte membrane. Of the protons and electrons, only protons can selectively pass through an electrolyte membrane called a proton exchange membrane and can reach a cathode, and electrons move through GDLs (conductive layers) and separators to reach the cathode.
Hydrogen ions that reach the cathode through the electrolyte membrane and electrons that reach the cathode through the separators combine with oxygen contained in air that is supplied to the cathode by an air supply unit, thereby producing water. At this point, movement of hydrogen ions induces an electric current that flows along an external wire, and heat, aside from the water, is also concomitantly produced as a byproduct.
As for a fuel cell system, a fuel cell stack deteriorates in performance thereof due to a variety of causes. Therefore, a technology for determining the causes of deterioration in performance of a fuel cell stack is needed. Potential causes of the deterioration in the performance of a fuel cell stack include flooding, drying, and contamination of the fuel cell stack.
For example, when the cause of deterioration in performance of a fuel cell stack is falsely determined as flooding when the actual cause is contamination of a fuel cell stack, an operation control of increasing a flow rate of air is performed to solve the problem of flooding, which aggravates dryness of a fuel cell stack. That is, an appropriate measure is not taken, thus failing to solve the problem of the fuel cell stack, and possibly creating new problems. Therefore, it is important to accurately and precisely diagnose a state of a fuel cell stack to determine the causes of deterioration in performance of a fuel cell stack so that appropriate measures to solve a problem of the fuel cell stack can be taken when a problem occurs in a fuel cell system.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.